Iron is one of the 16 elements essential for plant growth. Iron combines with a number of electron transfer proteins or enzyme proteins, as heme iron such as cytochromes or non-heme iron such as ferredoxins which are involved in redox reactions or biosynthetic reactions in many plants such as photosynthesis, respiration, nitrogen assimilation, and nitrogen fixation.
Plants absorb iron as bivalent iron ions. When plants cannot absorb bivalent iron ions, the growth of shoots is hindered as in the case of yellowing of rice, etc., and productivity declines. In degraded paddy fields, the concentration of bivalent iron ions in the plowed soil is low. If fertilizers containing sulfate ions such as ammonium sulfate are heavily used in such plowed soil, the generated sulfide is unremoved as iron sulfide, and rice roots are damaged by hydrogen sulfide (for example, see Non-patent Literature (NPL) 1). For healthy plant growth, it is important to stably and sustainably supply bivalent iron ions to plants.
Various techniques for stably supplying bivalent iron ions to plants have been proposed. For example, Patent Literature (PTL) 1 describes a technique for long-term and stable supply of soluble iron by using an iron-ion supply material that includes: an iron oxide and/or metallic iron-containing substance; and an organic acid which is gluconic acid or glutamic acid.